Weeds with Laura Wyeth Part II: The Japanese Knotweed

Here's an inside look on the invasive Japanese knotweed's growth in the Hudson Valley and how to deal with it.

Most Hudson Valleyites know this invasive plant by sight if not by name. It grows in clumps by the side of the road and has bamboo-like canes, shield-shaped leaves that turn golden-yellow in the fall, and showy puffs of little white flowers in August. Depending on the site, it can be 5 feet tall or 15 feet tall. It's Japanese knotweed and...it's nothing personal. It's just doing what it's evolved over millions of years to do.

Why and how is it so successful at colonizing the most degraded sites? Why is it so hard to control? And why might we, nonetheless, cultivate fondness for it? For Laura Wyeth, a Hudson Valley horticulturist with a particular interest in the adaptability of weeds, Japanese knotweed (hereafter, "knotweed") is a rock star. For all its troublesome nature, she can't help but admire its talents.

And it could be worse: in England, where knotweed enjoys a longer, milder growing season than in the US, this plant is such a problem that numerous companies specialize in trying to eradicate it—and equally numerous lawyers litigate claims over it. "The menacing presence of the plant has caused neighbors to file lawsuits, banks to cancel mortgages, and homes to lose their value," Wyeth says, "whereas here in New York State, its ability to edge out native species is considered its greatest destructive power, and, as in Britain, it is illegal to sell or purchase knotweed."

The Invasive Plant's Guide to Success

Japanese knotweed evolved as a first responder on the volcanic slopes of Japan. "It is the first plant to appear after the lava has flown," Wyeth says. "It colonizes the bare rock and slowly builds soil, allowing other species to return. When it appears in habitats disturbed by humans, as it has all over Europe and North America, it is only following its tenacious nature."

Knotweed was brought to Europe and the US from its native Japan in the late 1800s. It was prized for its novel ornamental qualities as an herbaceous perennial. For reasons unclear, in the 1930s, it started to escape the garden gate and set about colonizing roadside drainage ditches and construction dumpsites as well as woodland margins and streambanks. Its hollow, arching canes can grow 3 inches in a day.

Knotweed is built to conquer. Wyeth explains that its rhizomes (underground spreading stems) send out runners in all directions so that, like bamboo, an individual knotweed plant can quickly grow into a dense thicket. "Its tissues have remarkable powers of regeneration; a piece of broken stem or rhizome the size of a thimble can resprout into a new plant, and this often happens when soil containing pieces of rhizome is transported," Wyeth says.

Consequently, knotweed is often seen growing on construction sites where topsoil has been brought in from elsewhere. "It soldiers on through crummy soil and can sprout through small pavement cracks and push its way through asphalt," she says.

"Try to dig one out, and three will take its place. Knotweed laughs at pesticides. Its only predators, a few insects and one or two fungal diseases, have not followed it from Japan, and so its spread is unhindered," Wyeth says. It is now considered highly invasive throughout the US, Canada, and much of Europe.

Folks have attempted to get rid of knotweed via different repeated combinations of herbicides, cutting back stems, and grubbing out root systems—but control of Japanese knotweed is widely acknowledged to be imperfect at best. The best thing is to scout your property and catch it early. Remove it and all its underground parts, let it dry out in the sun in a bio-safe place (e.g., under black plastic weighted down on asphalt or concrete), and then burn it.

For the love of the Valley, don't dispose of knotweed plant parts in the garbage, in the woods, or anywhere else, as this will seed new colonies. Much of knotweed's spread has resulted from people dumping it near water bodies, which allows new colonies to take hold all along the waterways.

Knotweed: A Love Story?

If you have an established patch of knotweed that you can only hope to contain, you may just have to embrace or, dare we say, celebrate it. Bees forage on the blossoms at a time in the growing season (August into September) when there's not a lot of other options, and the resulting amber-colored honey is tasty.

The young shoots of the plant are edible in spring and taste pleasantly tangy when cooked, much like those of its cousin, rhubarb. You can find recipes online for strawberry-knotweed pie, knotweed wine, gazpacho, chutney, vodka, fruit leather, pesto. "In spring, I like to make a knotweed crumble, or add cooked young knotweed shoots to a salad to give it tang," Wyeth says. "You definitely always need to cook the young shoots, though."

As with most illicit substances, there's money to be made in knotweed. Wyeth explains that the stems and leaves are high in resveratrol, the same compound found, famously, in red wine. Resveratrol has been shown to reduce lipid levels in mice, and some studies suggest that it may have protective effects against cardiovascular and neurological decline, or even anti-cancer properties, in humans.

Despite the lack of strong evidence, resveratrol has been touted by many writers of health articles and makers of herbal supplements as a red wine-based anti-aging wonder drug. "Resveratrol tablets and tinctures are usually marketed with appealing images of grapes and wine, but in fact, the majority of resveratrol supplements on the market are, far more lucratively, isolated from Japanese knotweed," Wyeth says.

Allelopathy: Plants Trying to Murder Each Other

Knotweed has its own reasons for making resveratrol—it's a component of its chemical defense system. Plants engaging in chemical warfare are demonstrating allelopathy (allelo = "one another" and pathy = "disorder"). The most commonly recognized example of this is the way that the leaves, stems, fruit, buds, and roots of black walnut trees release a chemical compound, juglone, that inhibits the growth of most anything else in the vicinity. Knotweed practices its allelopathy with resveratrol and several other compounds.

Resveratrol is a phytoalexin, a compound produced in the tissues of the plant in response to microbial attack. Agricultural research has found extract of knotweed to be an effective antifungal agent, protecting crops such as tomatoes, wheat, and cucumbers from various fungal diseases—most notably, powdery mildew, gray mold, and rust. Knotweed's naturally-induced phytoalexins inhibit the formation of spores in pathogenic fungal species, and the extract, when sprayed on certain crops, confers the same benefit. As it is plant-based and minimally toxic to mammals, is considered safe for organic farming.

"The tale of knotweed is a microcosm for invasive plants generally," Wyeth says. "From some vantage points, it's a bad actor, but from others, it has a myriad of redeeming qualities."

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